Forced air return duct question

Ok I'm building an attic playroom, total finished out space will be around 420 sq ft. (room size 30x14) my question is, currently the return air flexible ducting is two 12" and two 8" for the downstairs. The two that are 12" come from the same area almost. So I want to combine those two ducts into one to the air box, so I will have more space behind the knee wall on the side of the room. Is this possible without taking away/reducing the air source to the air hadler /forced air unit. So in redneck terms, I want to combine two 12" return ducts into one duct to the heating cooling air unit and it will still be for the downstairs only. Upstairs will have a seperate unit installed. Also is it possible to combine the two 8" returns? I attached a phot for everyone to see what I working with. Thanks for the imput in advance.